1. Field of the Invention
The present invention relates to a semiconductive member, and a developing roll, a charging roll, and a transfer belt using the semiconductive member. The present invention also relates to an image forming apparatus using the developing roll, the charging roll, or the transfer belt.
2. Discussion of the Background
In the field of electric and electronic devices, resin materials which can precisely control static electricity have been demanded. For example, electrophotographic image forming apparatuses, such as copiers, facsimiles, and laser beam printers, form images through various processes including charging, irradiation, development, transfer, fixing, cleaning, and neutralization. Each of these processes requires precise control of static electricity.
In the charging process, a surface of a photoreceptor is evenly charged. In the irradiation process, an electrostatic latent image is formed on the charged surface of the photoreceptor by irradiation of light. In the development process, the electrostatic latent image is developed into a toner image that is visible. In the transfer process, the toner image is transferred from the photoreceptor onto a transfer material such as paper. In the fixing process, the toner image is fused on the transfer material by application of heat and pressure. In the cleaning process, residual toner particles remaining on the photoreceptor are removed. In the neutralization process, the charged photoreceptor is neutralized.
An electrophotographic image forming apparatus is typically equipped with a charging roll or belt, a developing roll, a toner layer thickness controlling blade, and a transfer belt. These members are required to have a semiconductive surface layer, more specifically a surface layer which has a volume resistivity of from 107 to 1011 Ω·m. For example, the charging roll, to which a voltage is applied, directly provides a photoreceptor with charge by direct contact with the photoreceptor. The developing roll frictions a toner supply roll so that toner particles are charged and the charged toner particles are adhered to a surface of the developing roll. The toner layer thickness controlling blade evens out the adhered toner particles on the developing roll. The toner particles fly to an electrostatic latent image on a surface of the photoreceptor by electric attraction force. The transfer belt is applied with a voltage having the opposite polarity to the toner particles so that an electric field is generated. The toner particles are transferred from the photoreceptor onto a transfer material by electrostatic force of the electric field.
As described above, various members in image forming apparatuses are required to have semiconductivity with an appropriately low volume resistivity. It is preferable that the volume resistivity is even at any point within a member. If the volume resistivity differs locally, high quality images cannot be produced. For example, if the volume resistivity distribution is uneven within a charging roll, a photoreceptor cannot be evenly charged, resulting in poor image quality.
A high voltage is repeatedly applied to the above members. Therefore, if the volume resistivity considerably varies upon application of a high voltage, high quality images cannot be produced reliably. Similarly, if the volume resistivity considerably varies upon variation in temperature and/or humidity, high quality images cannot be produced reliably. It may be possible to avoid effect of variation in temperature by warming up the apparatus, but it may be difficult to avoid effect of variation in humidity.
Various approaches have been proposed to control electric resistivity of polymer materials and moldings thereof. For example, one approach involves (1) applying an organic antistatic agent to the surface of a molding. Another approach involves (2) kneading an organic antistatic agent into a polymer material. Yet another approach involves (3) kneading a conductive filler such as a carbon black and a metal powder into a polymer material. Yet another approach involves (4) kneading an electrolyte in a polymer material.
However, the approach (1) has a disadvantage that the antistatic agent is likely to release when the surface of the molding is wiped or washed, resulting in short-term antistatic effect. In the approach (2), the organic antistatic agent is typically a surfactant or a hydrophilic resin. When a surfactant is used, electric resistivity and antistatic performance considerably vary upon variation in temperature and/or humidity because antistatic effect is provided by bleeding of the surfactant from the surface of a molding. When an antistatic agent is used, a large amount thereof is required to provide desired antistatic effect, which is likely to suppress good natures of polymers. In addition, there is a disadvantage that electric resistivity and antistatic performance considerably depend on humidity.
The approach (3) has been employed in various fields. For example, a typical charging roll is comprised of a cored bar which is covered with a semiconductive polymer composite material which is a polymer material into which a conductive filler is kneaded. However, such a semiconductive polymer composite material, which is a polymer material into which a conductive filler is kneaded, has a disadvantage that the volume resistivity distribution is very uneven. The degree of variation in volume resistivity is too large to put it into practical use. Additionally, such a semiconductive polymer composite material has another disadvantage that the withstand voltage is so low that it is not always suitable for intentional use such that high voltage is repeatedly applied. To achieve desired semiconductive level, a large amount of a conductive filler is required, which is likely to degrade molding processability of polymer composite materials or to increase hardness too much.
In the approach (4), as disclosed in Examined Japanese Patent Application Publication No. 63-14017, an alkali metal salt (i.e., an electrolyte) such as lithium chloride and potassium chloride is kneaded into a polymer material so that the electric resistivity is reduced owing to the presence of a metal ion such as Li+ and K+. Because inorganic metal salts such as alkali metal salts have poor compatibility with resins, they are likely to aggregate in the resins, resulting in poor electric resistivity. If the kneading temperature is increased or the kneading time is lengthened for the purpose of dissolving the aggregations in the resin, the problem may arise that the resin or the inorganic metal salts are decomposed, which results in destruction of mechanical properties and surface appearance. When a metal salt having deliquescence, such as a Li salt, is used in a large amount, the resulting polymer composite material may have hygroscopicity. In this case, the problems may arise that the volume resistivity considerably varies upon variation in humidity and the surface of the molding becomes sticky due to deliquescing substances of the metal salts.